Introduction to Bread Crumb Structure

Baked goods like bread and cake contain an internal part – crumb. It can be described by various metrics, that combined could be called crumb structure attributes. So what is crumb? It is a network of gas cells (pores) dispersed in a solid matrix of gelatinized starch and denatured protein. This porous crumb is essentially a set foam that forms during baking as gases expand and are trapped within the dough. The overall porosity (void fraction) of bread crumb is typically very high (bread crumb density often ranges around 0.25–0.35 g/cm³, meaning the crumb is 50 to 70% air¹). The size, shape, and distribution of these gas cells, as well as the thickness of the cell walls between them, define the crumb's morphology or "grain". Together, these attributes determine crumb appearance and texture. Crumb structure is a critical quality factor, strongly influencing both the sensorial experience and shelf-life of baked products.² For example, a fine, uniformly aerated crumb contributes to desirable softness and springiness, whereas a coarse or uneven crumb can adversely affect mouthfeel and acceptance. It was also identified during consumer acceptance study that bread crumb structure accounts for approximately 20% of the judgment of bread quality.³

The formation of the crumb's porous structure starts with the dough. Wheat flour, the principal ingredient, contains gluten-forming proteins (gliadin and glutenin) which, when hydrated and developed, produce a viscoelastic gluten network. This gluten network is critical – it traps the carbon dioxide gas bubbles generated by yeast fermentation, allowing the dough to rise without tearing.^4,5

During the baking process, a series of transformations set the crumb structure. As the dough heats, yeast or chemical raising agents produce gas (or existing gas bubbles expand), causing volume expansion. Proteins coagulate and starch gelatinizes around the expanding bubbles, which "sets" the structure of the crumb. Eventually, moisture evaporates and a crust forms on the exterior, which halts further expansion. The point at which the crust sets is crucial: if the crust forms too early, it constrains the interior and can lead to a denser crumb beneath the crust.⁶ Studies using controlled oven covers show that an imposed limit on expansion causes significant crumb densification – the earlier the constraint, the thicker the dense region just under the crust.⁷ Therefore, bakers often manipulate baking conditions (e.g. using steam in the initial baking stage to keep the crust flexible to ensure the crumb can expand to the desired openness.)

Several quantitative descriptors are used for crumb structure. The simplest is specific volume (loaf volume per unit mass), which relates inversely to average crumb density. Porosity (fraction of volume that is air) can be measured by techniques like gas pycnometry or by comparing crumb and true material densities.^2,6 However, porosity alone does not capture crumb quality; the pore size distribution is extremely important. For instance, two breads might have equal porosity, but one with many small cells will feel and appear much different than one with a few large voids. Image analysis of crumb slices is commonly used to measure cell size distributions, number of cells per area, cell elongation, etc. Cell wall thickness and cell connectivity (whether cells are isolated or interconnected by ruptures) are other key aspects: together with cell size, these determine the internal surface area and how quickly moisture or gases move through the crumb.⁶ In general, a well-developed crumb is characterized by a multitude of small, evenly distributed gas cells separated by elastic cell walls. Such a structure tends to maximize loaf volume while providing a soft yet resilient texture.

And in the next article, we'll discuss – how the dough's recipe and processing influence crumb structure development.